1. Field of the Invention
A system and method for detecting and monitoring tire and wheel imbalance for a wheeled vehicle.
2. Background Art
It is recognized in prior art teachings that dynamic behavior of a vehicle depends upon traction characteristics of the wheels and tires. Fluctuations in forces normal to the road surface at the tire road interface for each of the wheels will affect the traction characteristics of the tire. Known vehicle control systems for controlling vehicle yaw and roll characteristics may use wheel imbalance information to compensate for the adverse effects of wheel imbalance on the stability of the vehicle.
Sudden wheel imbalance may indicate, for example, tire effects such as loss of air pressure or tread loss. An improvement in vehicle dynamics made possible by detecting wheel imbalance, furthermore, may help avoid excessive tire wear and deterioration of vehicle wheel suspensions.
Although tire imbalance can be detected using spindle-mounted accelerometers or force transducers, Systems that use such imbalance detection techniques, of necessity, are relatively complex and costly.
Known methods for detecting out-of-roundness of tires, under-inflated tires, defective front-end alignment or off-center loads on the vehicle wheels are disclosed, for example, in U.S. Pat. No. 6,313,742. Instantaneous angular velocity of the wheels for each wheel of a vehicle is computed using anti-lock brake system data in the system of the ""742 patent and compared to an instantaneous computation of vehicle speed. Tire radius and tolerance data for the road wheels are used to compute rotational velocity of the wheels as a function of vehicle speed and tire radius data. The estimated velocity is compared to the computed rotational velocity to determine whether updated tolerances are respected. A pressurization error is computed if one wheel is characterized by tolerances that are outside a predetermined range.
An example of a monitoring system for measuring imbalance of vehicle tires using acceleration signals from an acceleration sensor for the wheels is described in U.S. Pat. No. 6,278,361.
The invention comprises a method for monitoring tire imbalance that does not require the use of accelerometers or force transducers. It instead uses wheel speed sensors that readily are available on vehicles equipped with automatic braking systems. This feature makes it possible to reduce the complexity of imbalance detection since hardware normally available with vehicles having automatic braking systems to improve wheel braking can be used for the additional purpose of providing control system input data to detect and monitor wheel imbalance.
The invention comprises a method for monitoring and measuring wheel speeds for vehicle road wheels and for computing an instantaneous dynamic rolling radius of each wheel using wheel speed data, vehicle speed, steering angle and yaw rate. A dynamic imbalance strength estimation then is computed using the rolling radius information. Based on the dynamic imbalance strength estimate for each wheel, the dynamic imbalance strength magnitude is computed. A tire imbalance logic then may be used, for example, to develop signals to activate a vehicle stability controller and to notify the driver of a wheel imbalance.